The species is often referred to as 'Lucy', after the nickname given to the most complete fossil A. afarensis skeleton to be discovered at the site.

The researchers say their study, published today in Science, should help resolve a long-standing debate over whether A. afarensis walked in much the same way as modern humans, or moved in some transitional way between that and the quadrupedal gait and tree-climbing behaviour of the great apes.

"The transition to full-time terrestrial bipedality is a hallmark of human evolution," the researchers write.

The earliest human ancestor well represented in the fossil record is Ardipithecus ramidus, from 4.4 million years ago.

The feet of Ardipithecus retain many characteristics of tree-dwelling ancestors, including the divergent and mobile big toe: A feature which modern apes have retained.

The researchers say their find, a seven-centimetre long fossilised mid-foot-bone, or metatarsal, proves that around one million years later, Lucy had stiff arched feet similar to those of humans.

Human foot unique among primates

The human foot is unique among primates. It's made up of two structural arches, one running along the foot and one across it. The metatarsals form the structure of the arches, which are supported by muscles in the sole of the foot.

Lead author of the paper, Dr Carol Ward of the University of Missouri, says the structure of the human foot is fundamental to the way we walk.

When the foot pushes off the ground the arches give leverage, and they absorb shock when the sole of the foot meets the ground at the end of each step.

"The development of arched feet was a fundamental shift toward the human condition, because it meant giving up the ability to use the big toe for grasping branches," says Ward.

"Our ancestors had finally abandoned life in the trees in favour of life on the ground."